A new and highly virulent outbreak of data-scrambling software — apparently sown in Ukraine — caused disruption across the world Tuesday. Following a similar attack in May , the fresh cyber-assault paralyzed some hospitals, government offices and major multinational corporations in a dramatic demonstration of how easily malicious programs can bring daily life to a halt.

Ukraine and Russia appeared hardest hit by the new strain of ransomware — malicious software that locks up computer files with all-but-unbreakable encryption and then demands a ransom for its release. In the United States, the malware affected companies such as the drugmaker Merck and Mondelez International, the owner of food brands such as Oreo and Nabisco.

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Its pace appeared to slow as the day wore on, in part because the malware appeared to require direct contact between computer networks, a factor that may have limited its spread in regions with fewer connections to Ukraine.

The malware's origins remain unclear. Researchers picking the program apart found evidence its creators had borrowed from leaked National Security Agency code, raising the possibility that the digital havoc had spread using U.S. taxpayer-funded tools.

"The virus is spreading all over Europe and I'm afraid it can harm the whole world," said Victor Zhora, the chief executive of Infosafe IT in Kiev, where reports of the malicious software first emerged early afternoon local time Tuesday.

In Ukraine, victims included top-level government offices, where officials posted photos of darkened computer screens, as well as energy companies, banks, cash machines, gas stations, and supermarkets. Ukrainian Railways and the communications company Ukrtelecom were among major enterprises hit, Infrastructure Minister Volodymyr Omelyan said in a Facebook post .

The virus hit the radiation-monitoring at Ukraine's shuttered Chernobyl power plant, site of the world's worst nuclear accident, forcing it into manual operation.

Multinational companies, including the global law firm DLA Piper and Danish shipping giant A.P. Moller-Maersk were also affected, although the firms didn't specify the extent of the damage.

Ukraine bore the brunt with more than 60 percent of the attacks, followed by Russia with more than 30 percent, according to initial findings by researchers at the cybersecurity firm Kaspersky Lab. It listed Poland, Italy and Germany, in that order, as the next-worst affected.

In the U.S, two hospitals in western Pennsylvania were hit; patients reported on social media that some surgeries had to be rescheduled. A spokeswoman for Heritage Valley Health System would say only that operational changes had to be made. A Wellsville, Ohio, woman at one of its hospitals to have her gallbladder removed said she noticed computer monitors off and nurses scurrying around with stacks of paperwork.

Security experts said Tuesday's global cyberattack shares something in common with last month's outbreak of ransomware, dubbed WannaCry . Both spread using digital lock picks originally created by the NSA and later published to the web by a still-mysterious group known as the Shadowbrokers.

Security vendors including Bitdefender and Kaspersky said the NSA exploit, known as EternalBlue, lets malware spread rapidly across internal networks at companies and other large organizations. Microsoft issued a security fix in March, but Chris Wysopal, chief technology officer at the security firm Veracode, said it would only be effective if every single computer on a network were patched — otherwise, a single infected machine could infect all others.

Bogdan Botezatu, an analyst with Bitdefender, compared such self-spreading software to a contagious disease. "It's like somebody sneezing into a train full of people," he said.

Ryan Kalember, a security expert at Proofpoint, said one reason the attacks appeared to be slowing down was that the ransomware appears to spread only when a direct contact exists between two networks — such as when a global company's Ukraine office interacts with headquarters.

But once it hits a computer on a network, it spreads quickly, even among computers that have applied the fix for the NSA exploit.

"It's more harmful to the organization that it affects, but because it's not randomly spreading over the internet like WannaCry, it's somewhat contained to the organizations that were connected to each other," Kalember said.

Botezatu said the new program appeared nearly identical to GoldenEye, a variant of a known family of hostage-taking programs known as "Petya." It demanded $300 in Bitcoin.

Unlike typical ransomware, which merely scrambles personal data files, the program wreaking havoc Tuesday overwrites a computer's master boot record, making it tougher to restore even a machine that has been backed up, said Kalember.

It may have first spread through a rogue update to a piece of Ukrainian accounting software called MEDoc, according to tweets by the country's cyberpolice unit. It said a rogue update seeded the infection across Ukraine. In a lengthy statement posted to Facebook, MEDoc acknowledged having been hacked.

The motives of those behind the malware remain unknown. Ukraine has been a persistent target of pro-Russian hackers, who are blamed for twice shutting down large swaths of its power grid in the dead of winter and sabotaging its elections system in a bid to disrupt May 2014 national elections.

Emails sent Tuesday to an address posted to the bottom of ransom demands went unreturned. That might be because the email provider hosting that address, Berlin-based Posteo, pulled the plug on the account before the infection became widely known.

In an email, a Posteo representative said it had blocked the email address "immediately" after learning that it was associated with ransomware. The company added that it was in contact with German authorities "to make sure that we react properly."